This invention pertains to solar cells and, more particularly, to a photovoltaic module framing system.
Conventional photovoltaic module framing systems use different frames to mechanically hold and mount the module to a support surface, such as a roof, or to a rack, and use a separate electrical system to electrically connect the photovoltaic modules. Most prior framing systems use an auxiliary bracket or beam to mechanically support and reinforce the framing system. This can be expensive, awkward, and ugly. Furthermore, conventional, photovoltaic electrical module framing systems usually require junction boxes and sometimes conduits which need to be attached to the framing system. Junction boxes are usually not aesthetically pleasing to consumers and are often not conveniently located. Furthermore, separate electrical ground conductors usually need to be connected to conventional photovoltaic systems, as well as expensive interconnection hardware.
Some prior framing systems have inwardly facing flanges with separate junction boxes. Inwardly facing flanges require that additional mounting fasteners be installed separately from the photovoltaic modules. This is an inconvenient process when attaching photovoltaic modules to a surface. In other prior framing systems, adjacent modules abut against each other and their plugs snap into sockets or other plugs in the sides of adjoining modules. As a result of thermal expansion, the plugs move in and out of electrical contact making bad contacts and sometimes cause electrical arcs across the modules which can start electrical fires. Furthermore, such an arrangement can corrode the plugs, cause system failures and result in unsatisfactory performance.
In order to install many conventional modules on a roof, the modules are commonly mounted to a larger frame, such as a structural rack, which is directly fastened to the roof on top of the shingles. Alternatively, the structural rack can be fitted into roofjacks and fastened to the roof on top of the shingles. There are many disadvantages to such prior conventional framing systems.
Photovoltaic modules which are manufactured using glass as a substrate or superstrate or as both surfaces have a high weight per unit area and are relatively fragile. Although large photovoltaic modules can be desirable, large modules are heavier, bulkier and more cumbersome. With four modules bolted to a structural frame in a sub-array, the sub-arrays are heavy and can require lifting equipment to elevate and position the four modules and frame on a roof.
The design and fabrication of the structural racks and roof jacks is usually left to the installer and can involve considerable labor and material cost. Commercially available, pre-made structural racks are also expensive. Positioning, assembling and fastening conventional modules to structural racks is labor intensive. It is also cumbersome to position and fasten the structural racks to the roof; or to lay out and fasten the roof jacks to the roof and the structural rack to the roof jacks. Clearances, gaps or spaces are often left between modules to permit the installer to place fasteners between the modules or sub-arrays. The appearance of some conventional module frames and spaces between modules and sub-arrays are regarded by many homeowners as aesthetically unpleasing on a finished roof. Such spaces also tend to collect leaves, twigs and other debris. The standard conventional picture frame-style or traditional external framing of the module can add considerable cost in material and labor to the finished photovoltaic module.
Roof-integrated installations have been tried by replacing certain portions of the roof construction with the photovoltaic modules themselves. This roof-integration effort has been attempted through approaches similar to glazing methods for skylights, but such methods have resulted in additional costs which are often higher than the amounts saved in replacing portions of the roof, therefore defeating their purpose.
Another drawback of roof-integrated photovoltaic module designs and conventional module framing systems is that they usually do not allow for the free flow of cooling air beneath the photovoltaic modules. Higher temperatures in the modules can result in lower power output and lower voltage from photovoltaic modules. In crystalline silicon cells, this temperature increase can decrease power output significantly and could lower the module voltage to a level which could not effectively charge batteries or produce the desired power in amorphous silicon cells. If a higher design voltage is chosen to ensure battery charging in hot weather for this reason, the result can be a loss of power for the module in spring, autumn and winter weather, due to a mismatch between battery voltage and optimum module voltage.
Some prior roof integrated module and system designs have attempted to cool the modules by utilizing glazing procedures to install the modules over roof framing members with the back surface of the modules open to an unheated attic space. This approach is not useful for insulated roofs. Furthermore, unheated attic spaces will have a much higher air temperature than the outside air, due to solar heat input from the roof, and because there is less air movement underneath the roof compared to the outside air. Fans can be used to help circulate air below the modules, but the use of fans is expensive and drain power.
It is, therefore, desirable to provide photovoltaic module framing system which overcome most, if not all, of the preceding problems.